i tape it down to my kitchen table or the pool table depending on how big the quilt is.
it helps to keep everything really smooth and even.
i also use post it notes to help with registration marks when i want to line up certain things on the quilt back with elements on the quilt top.
i also sewn on my hanging sleeve while i'm quilting the sandwich and i line the bottom of the sleeve so that it falls in a border seam. this way when i stitch in the ditch i'm sewing down the bottom of the hanging sleeve.
then when i sew on the binding i'm sewing down the top of the hanging sleeve.
the post it notes help me keep track of all these landmarks when i'm laying down the other layers of the quilt sandwich and i can line everything up.
the piece of wood that i lay over the table to baste is also marked every 1/4 each but i find the post it notes to work better - i would forget the measurement mark - so now i just put a post it on the location.